The present invention is directed towards a process for the concentration of aqueous hydrogen peroxide of concentration cE (H2O2 starting material) in order to obtain hydrogen peroxide of concentration cP (H2O2 product), cP being equal to or greater than 90 wt. %, by means of discontinuous crystallization.
In the known processes for the preparation of hydrogen peroxide, such as the anthraquinone cyclic process, electrolytic processes and methods of direct synthesis, hydrogen peroxide is obtained in the form of an aqueous solution. In order to convert such solutions into marketable products, the preparation is usually followed by concentration of the solution by distillation. By means of distillation it is possible to obtain aqueous hydrogen peroxide solutions having a content of up to approximately 90 wt. % hydrogen peroxide. Cost and safety considerations stand in the way of further concentration by distillation.
More highly concentrated hydrogen peroxide, that is to say products having an H2O2 content in the range of from equal to/greater than 90 wt. % to approximately 100 wt. %, are increasingly gaining importance commercially owing to their high energy content. It is known that hydrogen peroxide having a content up to approximately 100 wt. % is obtainable by repeated recrystallization. Fundamental aspects regarding the crystallization of hydrogen peroxide, as well as the solid-liquid phase diagram of the hydrogen peroxide/water system, are known from the reference book xe2x80x9cHydrogen peroxidexe2x80x9d by Walter C. Schumb et al., Reinhold Publishing Corp. (1955), p. 210-220. According to that book, a prominent property of highly concentrated aqueous hydrogen peroxide is the tendency to extreme supercooling, which makes concentration by crystallization more difficult. According to Schumb et al. (page 215), crystal formation is not affected in a reproducible manner either by stirring or scraping or by the cooling rate. However, freezing of supercooled highly concentrated hydrogen peroxide is induced by seeding with hydrogen peroxide crystals. The low efficiency of fractional batch crystallization is regarded as being a disadvantage of concentration by crystallization (Schumb et al., p. 215), since an increase in concentration of only approximately 2% and never more than 4% is achieved per crystallization stage. That problem is evidently the result of the high boundary solubility of water in crystallized hydrogen peroxide, which is also shown in the phase diagram (Schumb, page 211).
DE-PS 10 41 479 teaches an improved process for the concentration of aqueous hydrogen peroxide solutions. According to that process, an H2O2 starting material is cooled in a rectifying column to such an extent that there forms a 2-phase system consisting of a solid crystal phase and a liquid mother liquor, which phases differ according to the equilibrium with regard to the hydrogen peroxide concentration. The temperature along the rectifying column is so controlled that it increases slowly from one end to the other. Owing to the higher density of the crystallized hydrogen peroxide, the crystals and the mother liquor flow in a countercurrent relative to one another. A disadvantage of that continuous suspension crystallization process is the high technical outlay required to carry it out. Such a technical outlay only appears justified if very highly concentrated hydrogen peroxide is required regularly and/or in large amounts. However, the process is particularly uneconomical when the very highly concentrated hydrogen peroxide is required only periodically and/or in small amounts. In addition, scale-up poses problems.
Accordingly, an object of the present invention is to provide a simple process for the preparation of very highly concentrated hydrogen peroxide, that is to say hydrogen peroxide having a content in the region of equal to and, especially, greater than 90%, which process can be carried out batchwise by personnel with little training.
Another other object of the invention is to enable carrying out the process in simple technical apparatuses with little outlay in terms of control.
A still further object is to be able to scale up the process in a simple and reliable manner, in order to be adaptable to an increasing requirement.
The above and other objects can be achieved by the process according to the invention, which is based on the principle of layer crystallization with a subsequent sweating operation and which is carried out batchwise.
There has been found a process for the concentration of aqueous hydrogen peroxide of concentration cE (H2O2 starting material) in order to obtain hydrogen peroxide of concentration cP (H2O2 product), cP being equal to or greater than 90 wt. %, comprising discontinuous crystallization, which process is characterized in that a cooling surface arranged in a crystallizing vessel and wetted at least partially with hydrogen peroxide having a concentration of at least 70 wt. %, especially having a concentration in the range from cE to cP, is cooled by means of a cooling medium located at its rear side, at a cooling rate of at least 1 K/s, from a temperature in the region of the melting point of the hydrogen peroxide used for wetting to a temperature at which seed crystals form in the wetting medium.
The vessel is filled with H2O2 starting material having a temperature below the melting point of the H2O2 seed crystals. The temperature of the cooling medium is lowered to a value in the range from below the melting point of the H2O2 product to xe2x88x9250xc2x0 C. and, starting from the seed crystals, a crystal layer is allowed to grow during the temperature reduction and, if required, additionally after the temperature has been reached, uncrystallized hydrogen peroxide is then removed from the vessel. The crystal layer is subjected to a sweating operation in which the temperature of the cooling medium is increased in the course of from 0.2 to 20 hours to a value in the range from 10 K below to 5 K above the melting point of the H2O2 product. Hydrogen peroxide that is sweated out is separated off and crystalline hydrogen peroxide of concentration cP is obtained.
Surprisingly, the process according to the invention allows an H2O2 starting material having an H2O2 content of about 90 wt. %, especially 90xc2x12 wt. %, to be concentrated in one step to an H2O2 product having a content in the range of approximately from 97 to 99 wt. % or above.
The process according to the invention features the following essential steps:
1. formation of seed crystals on a cooling surface that has previously been wetted with highly concentrated hydrogen peroxide;
2. filling the vessel in which the cooling surface is located with the H2O2 starting material that is to be concentrated, the temperature of which is below the melting point of the seed crystals located on the cooling surface;
3. lowering the temperature of the cooling surface by reducing the temperature of the cooling medium, hydrogen peroxide having a higher concentration than that of the H2O2 starting material crystallizing on the cooling surface, starting from the seed crystals, during the cooling operation and the layer thickness of the crystal layer increasing;
4. discharging the uncrystallized aqueous hydrogen peroxide of reduced concentration (=mother liquor) from the crystallizing vessel;
5. carrying out a so-called sweating operation, in which the temperature of the cooling medium and hence of the cooling surface is slowly increased, the H2O2 concentration in the crystal layer increasing and less concentrated hydrogen peroxide dropping off as a melt.
6. The H2O2 product can be obtained by melting the crystal layer located on the cooling surface. After melting of the crystal layer, the cooling surface is again wetted with very highly concentrated hydrogen peroxide and can again be fed in that form to the first step of the process according to the invention.